1. Field of the Invention
The present invention relates to a multistage supercharging system of a reciprocating engine for an aircraft. More particularly, the invention relates to a multistage supercharging system used for a reciprocating engine of an aircraft capable of flying at an altitude of 25 Km or higher for a long time.
2. Description of the Related Art
In recent years, for various research projects (such as investigations of the behavior of the earth's atmosphere), aircraft capable of flying at an altitude of 25 Km or higher for a long time have been developed. The propulsion systems used for such aircraft include jet engines and reciprocating engines with multistage turbochargers. These systems are described in more detail below.
Typical jet engines include axial-flow compressors of multistage structure. In this type of axial-flow compressor, when the Reynolds number of a cascade of compressor blades is lowered by an increase in altitude, air flow separation occurs. This air flow separation influences the cascade of compressor blades at a back stage side, thereby lowering the efficiency of the compressor.
As is apparent from FIG. 8, which shows the relationship between altitude and Reynolds number, the critical Reynolds number of the axial-flow compressor is Re=50,000, and the corresponding altitude is about 20 km. Because of this relationship, if an aircraft flies at an altitude higher than about 20 km, the compressor does not work normally.
In addition, because jet engines quickly consume fuel, long flights at high altitudes with jet engine propelled aircraft cannot be obtained.
The following types of compressors can be used in jet engines: axial-flow compressors, centrifugal compressors, and oblique-flow compressors. The characteristics of air flow separation when the Reynolds number is lowered in these compressors are as follows:
(1) Because axial-flow compressors have a multistage structure, air flow separation at an early stage in the compressor influences the later stages. PA1 (2) Air flow separation occurring at an impeller of a centrifugal compressor is again attached by centrifugal force. PA1 (3) Oblique-flow compressors show characteristics equivalent to centrifugal compressors.
Thus, when the Reynolds number influence on compressor efficiency is taken into consideration, it is apparent that centrifugal compressors or oblique-flow compressors must be used for propulsion systems of high altitude aircraft flying at an altitude of 25 km or higher.
In reciprocating engines, engine output is lowered by the decrease of air density at high altitude. But, if the pressure of the engine intake air is boosted up to a pressure comparable to atmospheric pressure at ground level by using a turbocharger, the output of the engine can be increased. In addition, because reciprocating engines have a small fuel consumption rate, longer flights becomes possible with a limited amount of loaded fuel.
As shown in FIG. 9, the maximum pressure ratio value in existing turbochargers is about 4.5:1 for an aircraft.
From this data, it follows that using a reciprocating engine equipped with a one stage turbocharger, an engine output comparable to that at ground level can be obtained up to an altitude of about 11 km, at which altitude the atmospheric pressure becomes about 1/4 of that at ground level.
When a reciprocating engine equipped with a two stage turbocharger is considered, an aircraft has an altitude limit of about 21 km, at which altitude the atmospheric pressure becomes about 1/(4.5.times.4.5)=1/20 of the atmospheric pressure at ground level.
Thus, to enable flight at an altitude of 25 km or higher, it is necessary to combine at least three turbocharger stages with the reciprocating engine.
A technical discussion concerning a reciprocating engine equipped with three stage turbochargers to enable flight at a high altitude follows.
First, a reciprocating engine with three stage turbochargers made by GROB Co., which was mounted on aircraft STRATO2C made in Germany and with past records of flight at an altitude of 24 km, will be described.
As shown in FIG. 10, the turbochargers of this engine include a high pressure stage turbocharger 1, a two-axis low pressure stage turbocharger 2, and an intermediate pressure stage turbocharger 3. When the aircraft flies at an altitude of lower than 7 km, only the high pressure stage turbocharger 1 is used. When the aircraft flies at an altitude of higher than 7 km, all of the turbochargers 1, 2, and 3 are operated to supply a supercharging pressure.
In the ERAST (Environmental Research Aircraft and Sensor Technology) program developed by NASA, a reciprocating engine with three stage turbochargers has been developed which is capable of flight at an altitude of about 25 km.
As shown in FIG. 11, turbines 11, 12, and 13 of the high pressure stage, the intermediate pressure stage, and the low pressure stage turbochargers, respectively, are connected to each other through one pipe 14. By this arrangement, the exhaust energy of the engine drives all of the turbines 15, 16, and 17 of the high pressure stage, the intermediate pressure stage, and the low pressure stage turbochargers, respectively, to produce supercharging. It appears that when a higher supercharging pressure is obtained than that needed, a throttle 18 of the engine and a wastegate valve 19 are operated so that the supercharging pressure is controlled.
In the reciprocating engine with the three stage turbochargers made by GROB Co. (see FIG. 10), the high pressure stage turbocharger 1 bypasses exhaust energy of the engine through an exhaust wastegate valve 4 to an exhaust pipe 5, and supercharging pressure obtained by a high pressure stage compressor is bypassed to an exhaust pipe 7 through a high pressure stage bleed valve 6, so that the supercharging pressure is controlled.
When control is accomplished by bypassing the supercharging pressure obtained by the compressor to an exhaust pipe in this manner, that is, by throwing away the supercharging pressure via the exhaust pipe, the compressor operates more than necessary, thereby resulting in low system efficiency.
In addition, in the low pressure stage and intermediate pressure stage turbochargers 2 and 3, respectively, because one pipe 8 connects the turbine outlet of the high pressure stage turbocharger 1 and the low pressure stage and intermediate pressure stage turbines, the low pressure stage and intermediate pressure stage turbines also are driven by the exhaust energy of the high pressure stage turbine. Therefore, because the low pressure stage and intermediate pressure stage compressors are always driven irrespective of whether supercharging is necessary, the driving loss is large.
Moreover, this system is designed such that when a higher supercharging pressure than that needed is obtained by the low pressure stage and intermediate pressure stage compressors, the excess pressure is bypassed to the exhaust pipe by a low pressure stage bleed valve 9L and an intermediate pressure stage bleed valve 9I in a manner similar to that used with the high pressure stage turbocharger. This results in a low system efficiency.
In the NASA reciprocating engine with the three stage turbochargers (see FIG. 11), because pipes 14 connect the turbines, the exhaust energy of the engine always drives each of the high pressure stage, intermediate pressure stage, and low pressure stage turbines 11, 12, and 13, respectively. Therefore, the low pressure stage, intermediate pressure stage, and high pressure stage compressors 15, 16, and 17 are always operated, resulting in a large driving loss.
In a low altitude flight, it is not necessary to operate all of the three stage turbochargers. However, because this system always drives all of the turbochargers, the supercharging pressure required at low altitude is divided among the turbochargers. Because of this arrangement, it is impossible to use the compressors at each stage in a high efficiency range based on the flight altitude, and therefore, the efficiency of the whole system is lowered.
Moreover, in this system, because all of the turbines are driven together, a time delay occurs in the response of the turbines and compressors. For this reason, when an abrupt pressure change occurs at the intake or exhaust of the engine, for example, from a gust of wind or the like, there is a possibility that the operation of the engine will become unstable because of this response time delay, thereby making control difficult.